In a conventional electronic voice communication system, each user typically communicates by means of an audio input device, e.g., a microphone that may be held close to the user's mouth or mounted on a wall or console, and an audio output device, e.g., a speaker mounted in a handset, a headset worn by the user or mounted on a wall or console nearby. Such communication systems typically include telephone, speakerphone, intercom, mobile radio systems and many specialized communication systems such as aircraft communication systems which generally include both an intercom for communication within the aircraft, e.g., between a pilot and co-pilot, and a radio-frequency transceiver ("radio") for external communication, e.g., with an airport tower.
Such communication systems must often deal with notoriously noisy and busy environments, where any received messages can be difficult to comprehend because of ambient environmental noise, and where users often do not have their hands free to operate, e.g., squelch controls on the systems to reduce the background noise.
One prior art arrangement for reducing background noise involves the use of voice-activated control circuits for hands-free control of the communication system. Thus, if no one speaks, the microphone does not transmit; if someone speaks, however, the microphone is turned "on" automatically, and the system "listeners" can hear the spoken words. Known versions of such voice-activated circuits find application in many types of commercial products, such as baby-minders and tape-recorders.
Voice activated control circuits typically incorporate voice detection circuits (sometimes called "voice detectors") and voice-operated switches ("VOS") that control communication in response to control signals from the detectors. On detection of the presence of a human voice, the VOS is activated so as to turn "on" the product.
The voice detector must distinguish voice from other sounds, i.e., noise, in order to prevent needless and undesirable VOS activation. Conventionally, voice is distinguished from noise by identifying sudden increases in audio magnitude that would indicate the commencement of speech.
Typically, such a voice detector has an adjustment for setting a particular noise amplitude threshold, below which any detected audio signal will not activate the VOS, and above which any detected audio signal will activate the VOS. The threshold adjustment is sometimes factory-set, but can often be set by a user during operation of the system, e.g., by setting an externally-accessible control pot.
Unfortunately, a fixed, factory-set noise amplitude threshold can significantly compromise the operation of the detector. If the threshold is set too low, ambient noise levels can activate the VOS (i.e., "false-positive" activation), thereby causing the system to transmit mere noise. On the other hand, if the threshold is set too high, the VOS may not activate during some genuine voice transmissions (i.e., "false-negative" activation).
User-adjustment of the noise threshold, when provided, causes still other problems. More particularly, operation of the system can require that the user adjust the threshold at inconvenient times. For example, in voice-activated aircraft intercoms, threshold-adjustment can require the pilot to change the intercom settings during take-offs and landings. Moreover, since noise levels change frequently in many environments, the user often must re-adjust the threshold frequently. Such activities may be more than inconvenient; they can be serious distractions and annoyances for all concerned.